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In the past, most important organic chemicals were derived from coal. Nowadays, natural gas and crude petroleum provide an alternative source. The composition of crude petroleum varies according to its source It is a dark coloured, viscous liquid which consists mostly of alkanes with up to 40 carbon atoms, plus water, sulphur and sand It can be split up into fractions by fractional distillation Distillation separates the compounds according to their boiling point At each level a mixture of compounds in a similar boiling range is taken off Rough fractions can then be distilled further to obtain narrower boiling ranges Some fractions are more important - usually the lower boiling point ones High boiling fractions may be broken down into useful lower boiling ones - CRACKING CONTENTS PETROCHEMICALS
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Chapter21
Petrol
Crude oil
• Crude oil is a fossil fuel.• It was formed from the bodies of tiny sea
creatures which died millions of years ago.• Bacteria turned the bodies of these creatures
into crude oil and natural gas.
PetrolPetrol is formed by the fractional distillation of crude oil.
Some of the different fractions formed are:Petroleum Gas - Metacarpans added for smell.PetrolNaphta - Petrochemical IndustryKerosene - Aviation Fuel.Diesel.Lubricating Oil.
Octane NumberKnocking - early explosion due to petrol-air mixture. The petrol explodes
as it is compressed instead of being caused by a spark. (Auto-ignition).
Octane Number - Measure of the tendency of fuels to resist knocking.
The octane number of a fuel can be measured by comparing the performance of 2, 2-4 tri-methyl pentane (iso-octane) which has an octane number of 100 and Heptane which is not efficient and has an octane number of 0.
The shorter the alkane the higher the octane number.The more branched the chain the higher the octane number.Cyclic compounds have a higher octane number than straight chain
compounds.
Octane Number
Making PetrolTetra-ethyl lead -Reduces knocking. However it caused health problems
and is poisonous.
4 ways to increase octane number.1. Isomerisation. Branching caused by alkanes heated in the presence of a
suitable catalyst.2. Catalytic Cracking. Long chained hydrocarbons broken down into short
chain molecules for which there is great demand.Large alkanes form alkanes and alkenes.
3. Reforming (DeHydrocyclisation.) Reforming involves the use of catalysts to form ring compounds.-Hydrogen given off.
4. Adding Oxygenates. Three oxygen containing molecules, methanol, ethanol, and MTBE are added to petrol to increase its octane number.
MTBE stands for methyl tertiary butyl ether. (2-methoxy-2-methylpropane)
